Unshunted QOS Comparator for Qubit Readout

Abstract

State measurements of flux-based superconducting quantum bits (qubits) require the sensitive discrimination of small differences of magnetic flux, with measurement times short compared to the timescales of coherent quantum mechanical behaviour. The single-fluxquantum (SFQ) superconducting comparator may be the best device for this purpose. An SFQ comparator design using a quasi-one-junction SQUID (QOS) acting as the comparator junction has two advantages over earlier designs-increased sensitivity and the possibility to eliminate shunt resistors. Here we report the first measurements of the sensitivity of an unshunted QOS comparator. The gray zone of this device at 4.2K is 6 mΦo.

Single-Flux-Quantum Circuit Based Readout System for Detector Arrays by Using Time to Digital Conversion

“…QOS designs reported by other groups are mainly used in the analog to digital converter circuits or QUBIT read-out circuits and non-hysteretic operation is desired [10]- [13]. In order for the circuit to be non-hysteretic, it is necessary that the SQUID loop be non-hysteretic and should be designed to be less than 1.…”

Experimental Demonstration and Numerical Analysis of Microampere Gray Zone Width with Enhanced Operating Margin in Shunted Quasi-One Junction Superconducting Quantum Interference Device Comparators

“…13,14) The current sensitivity of QOS comparators has already been investigated by other researchers. 15,16) However, the QOS comparators used in these studies did not have shunt resistors to reduce thermal noise. As a result, these unshunted QOS comparators did not operate at high frequencies.…”

“…Further, the current sensitivity of the unshunted QOS comparators obtained in the numerical analyses did not concur with the experimental results. 16) The comparators used in large-scale readout circuits in multiple superconductor detector systems should have the following specifications: First, all the comparators should have sensitivities suitable to sense the current output of the detectors. The required current sensitivity should be less than 6 A.…”

“…19) All the studies conducted thus far have mainly focused on decreasing the sensitivity levels of QOS circuits. 15,16,20) However, from the viewpoint of practical applications, we focus on increasing the bias margin with acceptable sensitivity in this study.…”